Scott, I had very simlar problem like yours going from my mixer (yamaha 01v) to Yamaha Motif. I was using an un-balanced cable. swapped it out with a balanced cable and everything was fine.

Looking at the cable it looks like a normal stereo cable with tip, ring, sleeve configuration so I also thought that it is the same as standard stereo cable but when I asked the sales person at Sam Ash, where I bought my cables from, he told me that it is not the same. He did explain why but I can't rememeber.

I dont't know about your amp but I will be very surprised if Tyros had unbalanced connectors. I think all pro keyboards have balanced connectors.